The present invention relates to a combustion method in an industrial combustion system and, more particularly, to such a combustion method in which a combustion air to be fed to the combustion device is preheated by a high-cycle regenerative combustion system, so that the preheated combustion air is introduced into a combustion zone to generate and maintain a combustion reaction by means of the high temperature combustion air and a combustion fuel.
A regenerative combustion system has been known that has a combustor and a heat exchanger for heat recovery. Such a heat exchanger is brought into contact with a low temperature fluid which is combustion air, fuel gas or a mixture of the combustion air and the fuel gas, and a high temperature fluid which is exhaust gas generated through the combustion, such as flue gas, burnt gas or combustion gas. The heat energy possessed by the high temperature fluid is accumulated in the heat exchanger and transferred to the low temperature fluid. This type of heat exchangers can be practically used in a variety of plants or industrial furnaces such as a hot air oven for a blast furnace, a coke oven, and a glass melting furnace.
Such a combustion system may have, for example, a pair of burners which constitute a combustor, and a first regenerative heat exchanger and a second regenerative heat exchanger which are disposed in the respective combustion air introduction passages leading to the respective burners. The burners are switched to operate alternately and periodically at a predetermined cycle time such that, when the first burner operates to burn a fuel, the exhaust gas generated as a result of the burning is discharged through the other combustion air introduction passage associated with the second burner. Consequently, the heat energy of the hot exhaust gas is accumulated and conserved in the second heat exchanger as a result of exchange of heat between the exhaust gas and the second heat exchanger. The combustor is then switched so that the second burner is put into operation. During the operation of the second burner, combustion air is supplied through the second heat exchanger, which has been already heated, so as to be pre-heated before reaching the second burner.
In the known switching heat-regenerative combustion system, the cycle time at which the burners are switched is set to be a very long time, which impracticably lowers and degrades a temperature efficiency and a heat recovery efficiency. Further, the whole apparatus including the heat exchangers has to have a large scale in order to realize a great heat accumulation capacity. Under this circumstance, a system generally referred to as high-cycle regenerative combustion system (HRS) or high-speed switching regenerative combustion system has been proposed in recent years, which system is intended to have an improved temperature efficiency, as well as a reduced size, so as to eliminate the above-described drawbacks of the conventional system.
Meanwhile, the present applicant already has proposed, in the specification of Japanese Patent Application No. 2-415583 (Laid-Open No. 4-251190), a honeycomb type ceramic heat accumulator which serves as a heat-regenerative heat exchanger for use in a high-cycle regenerative combustion system of the kind as mentioned above.
The honeycomb type heat accumulator discussed in the above-mentioned specification has been constructed to meet the following three major design requirements:
(i) To set the net or substantial volume Vc per the apparent or gross volume V of the heat accumulator to be a large value, in order to enhance the heat accumulation capacity. PA1 (ii) To set the heat transmission area At per the apparent volume V of the heat accumulator to be a large value, in order to enhance the heat transmission rate. PA1 (iii) To set the pressure loss .DELTA.P of the fluid to be a small value.
In addition, the pitch or span of the cell walls and the thickness of the cell wall, i.e., the honeycomb pitch and the honeycomb wall thickness of the heat exchanger, are determined such that the multiplication product of the above-mentioned three factors, i.e., (Vc/V) by (At/V) by (1/.DELTA.P), substantially exhibits a maximum value. At the same time, the ratio P/b between the honeycomb pitch P and the honeycomb wall thickness b is preferably determined so as to range from 5 to 10, more preferably to be 7.5.
However, if an approach is made to attain a high temperature efficiency of a range from 0.7 to 1.0 in a combustion system of a conventional structure, an overall volume or bulk size of its heat exchanger is enlarged in a designed size or an actual size to an extent that it would be inapplicable to a practical use. For this reason, the overall size of the heat exchanger has to be limited to some extent; otherwise, necessary designed components have to be partially eliminated in spite of a degraded heat exchanging ability of the heat exchanger. In any event, the ability of heat exchanger has been limited to an order of approximately 0.6 at the greatest.
Thererfore, the conventional heat exchanger of a relatively low temperature efficiency allows the combustion air at an average ambient temperature around 20.degree. C. to be preheated by means of the sensible heat transferred from the combustion exhaust gas, so that the preheated air flow at a relatively low temperature is fed to a combustion zone in a furnace. In such a conventional arrangement, the normally available temperature of continuously preheated combustion air can be merely set to be 500.degree. C..about.600.degree. C. as an upper limit.
Meanwhile, a higher temperature of combustion air leads to a higher temperature in the combustion zone proportionally. In general, it is commonly known that the production rate of combustion products such as nitrogen oxides (NOx) increases as the temperature in the combustion zone rises. Therefore, under the combustion theory conventionally established or its relevant technical knowledge of those skilled in the art, a combustion reaction generated in a high temperature combustion air over 500.degree. C..about.600.degree. C. contradicts social requirements for further stringent restrictions on the exhaust emission level of combustion products such as nitrogen oxides (NOx) and an environmental or ecological regulations or standards for cleanness of waste materials or gases. Therefore, such a high temperature combustion condition is considered to be industrially impermissible. Thus, the preheat temperature of combustion air for a combustion system is generally limited to 400.degree. C..about.500.degree. C. at the highest, since a combustion air flow of a temperature over 500.degree. C..about.600.degree. C. is considered to be inapplicable to an industrial combustion system. Actually, even an effort has not been made yet to research or develop a combustion mode in such a high temperature combustion condition.
As regards an industrial furnace of a type in that a preheated combustion air in a temperature range of 400.degree. C..about.600.degree. C. is fed to a combustion system, it is technically confirmed that a flame once created in the combustion zone tends to be extinguished or blown out when the oxygen concentration or density of the combustion air is reduced down to 18% or less. However, it has not been known whether a flame itself can be continuously created and kept in force within a combustion air flow preheated to a temperature range higher than 500.degree. C..about.600.degree. C., or whether a combustion reaction itself can be normally maintained in such a condition without flame failure. Further, since the significance or meaning of research or development on such a high temperature combustion condition is not apparent, any substantial research or development has not been conducted yet. Still further, it is considered by those skilled in the art that, even if a flame in the combustion zone can be formed and maintained in such a high temperature air, a relatively large amount of nitrogen oxides (NOx) would be produced as described above, and therefore, that such a combustion mode in a highly preheated air is to be an impractical combustion mode which would be industrially inapplicable in view of the restriction requirements as to exhaust emission level and so forth in recent years.
Such being the case, research or development efforts in recent years are mainly directed to, e.g., improvement of a two-stage combustion method in which combustion air of a relatively low temperature or hydrocarbon fuel is stepwisely fed to a combustion zone.